Recovery of AZT by guanidine precipitation

ABSTRACT

A process for recovering 3&#39;-azido-3&#39;-deoxythymidine (AZT) or a salt thereof from a mixture of chemicals involving the reaction of the AZT with guanidine to form a salt precipitate. The salt precipitate is then acidified to pH of less than about 9 to yield free AZT, which can then be crystallized from solution. 3&#39;-Azido-3&#39;-deoxythymidine and its salts, also known as AZT and Zidovudine, is used in the treatment of AIDS patients.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation-in-part of application Ser. No. 07/717,831, filed Jun. 21, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a press for recovering nucleoside derivatives. More particularly, the present invention relates to a process for recovering 3'-azido-3'-deoxythymidine or derivatives thereof directly from the reaction mixture and from the mother liquor remaining from a general process for production thereof.

2. Brief Description of the Prior Art

Nucleoside derivatives having anti-viral activity are known. For example, U.S. Pat. No. 4,211,773 (Lopez et al) discloses pyrimidine nucleosides, specifically, 5-substituted-1-(2'-deoxy-2'-substituted-β-D-arabinofuranosyl) pyrimidine nucleosides which exhibit anti-viral effects. These compounds have the following general formula: ##STR1## wherein A may be, inter alia, an oxyalkyl group; B may be oxygen or sulfur; X may be, inter alia, a halogen; Y may be, inter alia, a halogen or a substituted or non-substituted amino group; Z may be methyne or nitrogen; and each of R¹ and R² may be, inter alia, hydrogen.

Further, as disclosed in U.S. Pat. No. 4,904,770, other thymidine derivatives have been reported to possess in vitro activity specifically against the AIDS virus. The compound 2',3'-dideoxy-2',3-didehydrothymidine(d4T) is such a thymidine derivative and has the following general formula: ##STR2## wherein each of X, Y and Z may be, inter alia, nitrogen or C--H and R⁴ may be OH or NH₂.

3'-Azido-3'-deoxythymidine, also known commercially as AZT or Zidovudine, is one of the most commonly known nucleoside derivatives having activity against the AIDS virus and thus, is useful in treating humans infected with the virus. Pharmaceutically basic salts of AZT as well as 5'-mono-, di- and tri-phosphates of AZT or basic salts thereof (i.e. derivatives thereof) are also useful in treating AIDS. As is well known in the art, 3'-azido-3'-deoxythymidine has the following chemical formula: ##STR3##

Various methods of producing and/or using zidovudine or related compounds such as the pharmaceutically basic salt thereof and the 5'-mono-, di- or tri- phosphate thereof are disclosed in J. Org. Chem. 38, 4299 (1973) (Glinski et at), Nucleosides and Nucleotides, 9, 629 (1990) (Watanabe et al), and in U.S. Pat. Nos. 4,724,232 (Rideout et al), 4,818,538 (Rideout et at), 4,818,750 (Rideout et at), 4,828,838 (Rideout et at), 4,833,130 (Rideout et at), 4,837,208 (Rideout et al), 4,847,244 (Rideout et at), 4,857,511 (Rideout et at), 4,874,609 (Rideout et at), 4,916,218 (Almond et al), the contents of each of which are hereby incorporated by reference.

Large scale manufacture of 3'-azido-3'-deoxythymidine based on one or more of the methods of production referred in the previous paragraph is known. The resultant crude crystalline 3'-azido-3'-deoxythymidine can be recrystallized to pharmaceutically acceptable purity (i.e. purity greater than 99.5%) by conventional techniques. Unfortunately, while these techniques may be useful to reduce the levels of impurities, they require multiple recrystallization steps and thus the efficiency of the recovery of pharmaceutical grade 3'-azido-3'-deoxythymidine is significantly less than theoretical, typically from about 30 to about 60 percent at most. For example, Glinski et al (see above) report a yield of only about 30 percent of theoretical. Since the cost of producing 3'-azido-3'-deoxythymidine can easily exceed U.S. $5000/kg, any yield of the final product (e.g. in pharmaceutically pure form) significantly less than theoretical is indicative of a relatively inefficient process. In light of this, it would be desirable to have an improved process for recovery of 3'-azido-3'-deoxythymidine or derivatives which results in an overall improved yield of the final product.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel process for the recovery of 3'-azido-3'-deoxythymidine or derivatives thereof from a mixture comprising, inter alia, zidovudine or derivatives thereof.

In copending application Ser. No. 07/717,83 1, there is disclosed novel anti-viral compounds and processes for production thereof. The novel anti-vital compounds have the following general formula: ##STR4## wherein X is selected from the group comprising hydrogen, methyl, trifluoromethyl, propenyl, fluoro, chloro, bromo and iodo.

In the development of these "dimer" anti-viral compounds, a process has been discovered whereby it is possible to produce enhanced yields of 3'-azido-3'-deoxythymidine or derivatives thereof. Specifically, in related application Ser. No. 07/717,831, a process disclosed for production of the "dimer" anti-vital compounds comprises the coupling reaction of a pyrimidine derivative and a 3'-azido-3'-deoxy-5'-O-substituted-pyrimidine compound. A preferred example of the latter compound is 3'-azido-3'-deoxythymidine. It has now been discovered that, in cases where the production of 3'-azido-3'-deoxythymidine forms a part of an overall process for producing the "dimer" anti-vital compound, substantial prevention of co-crystallization of the "dimer" anti-vital compounds can actually lead to enhanced yields of 3'-azido-3'-deoxythymidine. This offers a degree of control in the use of the present invention. Specifically, one can substantially prevent co-crystallization of the "dimer" anti-viral compounds which can lead to an enhanced yield of 3'-azido-3'-deoxythymidine and the increased percentage of the dimer in the mother liquor from the reaction may be recovered by any suitable means such as chromatographic techniques.

Accordingly, this aspect of the present invention provides a process for recovering 3'-azido-3'-deoxythymidine or a derivative thereof from a mixture of chemicals comprising 3'-azido-3'-deoxythymidine or a derivative thereof, the process comprising the steps of

(i) reacting 3'-azido-3'-deoxythymidine or a derivative thereof in the mixture with a reagent selected from the group consisting of guanidine, a combination of a base and a guanidine salt, and mixtures thereof, to precipitate a salt;

(ii) acidifying the precipitated salt to a pH of less than about 9 to produce substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof; and

(iii) recovering the substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof.

By using this process, the co-crystallization of the "dimer" anti-viral compounds is substantially minimized and results in enhanced yields of substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof.

The choice of guanidine salt is not particularly restricted and guanidine salts are generally known to those skilled in the art. Non-limiting examples of suitable guanidine salts include chloride, bromide, iodide, sulfate, phosphate, acetate, nitrate and carbonate. The preferred guanidine salt is chloride.

The term "base", as used herein with reference to Step (i), includes any compound which is capable of acting as a Lewis base (i.e. a substance that can donate a pair of electrons to the formation of a covalent bond). Suitable bases may be selected from the group consisting of alkali metal compounds, alkaline earth metal compounds, ammonium hydroxide and organic base compounds. Non-limiting specific examples of alkali metal compounds include: lithium hydroxide, potassium hydroxide, and preferably sodium hydroxide. Non-limiting examples of alkaline earth metal compounds include: magnesium oxide, magnesium hydroxide, calcium oxide, barium oxide, barium hydroxide, and most preferably calcium hydroxide. Non-limiting examples of organic base compounds include alkyl amines such as R¹ R² R³ N wherein R¹,R² and R³ may be the same or different and are selected from the group comprising C₁₋₁₀ alkyl groups and hydrogen. Preferred examples include methylamine, ethylamine, isopropylamine, n-butylamine, tert-butylamine, dimethylamine, diethylamine, diisopropylamine, triethylamine, piperidine and piperazine. Preferred organic bases include isopropylamine, n-butylamine, tertbutylamine, diethylamine, diisopropylamine and triethylamine.

DETAILED DESCRIPTION OF THE INVENTION

The present process is useful in the recovery of 3'-azido-3'-deoxythymidine or a derivative thereof from a mixture of chemicals comprising 3'-azido-3'-deoxythymidine or a derivative thereof (hereinafter referred to as "the 3'-azido-3'-deoxythymidine recovery process"). As used throughout the present specification, the terms "recovery" or "recovering" in the context of 3'-azido-3'-deoxythymidine or a derivative thereof are intended to mean that substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof are reclaimed from a source of substantially impure 3'-azido-3'-deoxythymidine or a derivative thereof. The term "pure zidovudine or a derivative thereof" is intended to mean that the 3'-azido-3'-deoxythymidine or derivative thereof is sufficiently pure to be considered by those skilled in the art to be pharmaceutical grade, i.e. at least about 99.5% pure. By analogy, the term "impure 3'-azido-3'-deoxythymidine or a derivative thereof" is intended to mean that the 3'-azido-3 '-deoxythymidine or derivative thereof is not sufficiently pure to be considered by those skilled in the art to be of pharmaceutical grade, i.e. less than about 99.5% pure.

The mixture of chemicals comprising 3'-azido-3'-deoxythymidine or a derivative thereof may be considered a source of impure 3'-azido-3'-deoxythymidine or a derivative thereof. Generally, the exact source of this mixture of chemicals is not restricted. For example, the mixture may evolve from a conventional process for the production of 3'-azido-3'-deoxythymidine such as that disclosed by J. Org. Chem. 38, 4299 (1973) (Glinski et at) and Nucleosides and Nucleotides, 9, 629 (1990) (Watanabe et at). In such a case, the present process may be used as a purification or isolation step in an overall process for the production of 3'-azido-3'-deoxythymidine or a derivative thereof. Alternatively, the present process may be utilized advantageously to reclaim 3'-azido-3'-deoxythymidine or a derivative thereof from the mother liquor of the purification step of the conventional process discussed above. In either of these embodiments, the overall yield of substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof which may be obtained using the present process may be as high as 75% or more for the two steps. This can translate into significant overall savings on the cost of conducting the overall process.

The term "derivative" as used herein in the context of a derivative of 3'-azido-3'-deoxythymidine is intended to cover compounds such as pharmaceutically acceptable salts of 3'-azido-3'-deoxythymidine. Non-limiting examples of such salt include alkali metal salts (e.g. sodium, potassium, etc.), alkaline earth salts, organic bases (e.g. amines, etc.) and ammonium salts. Other derivatives of 3'-azido-3'-deoxythymidine which are useful in the present process include the mono-, di- and triphosphates of 3'-azido-3'-deoxythymidine whose chemical formulae are, respectively: ##STR5##

In Step (i) of the present process, 3'-azido-3'-deoxythymidine or a derivative thereof may be reacted with guanidine to form a precipitated salt. Alternatively, and preferably, 3'-azido-3'-deoxythymidine or a derivative thereof is reacted with a combination of a base and a guanidine salt (preferably guanidine chloride) to form a precipitated salt.

In either case, the so-formed salt precipitates from most aqueous and non-aqueous solvents. Accordingly, the choice of solvent for use in Step (i) of the present process is not particularly restricted. Preferably, the solvent is polar, more preferably the solvent is selected from the group consisting of water, alcohols (e.g. methanol, ethanol and isopropanol) and ketones (e.g. methylisobutyl ketone and acetone). Most preferably, the solvent is water.

The manner in which 3'-azido-3'-deoxythymidine or a derivative thereof is reacted with guanidine in Step (i) of the present process is not particularly restricted. In one embodiment, 3'-azido-3'-deoxythymidine may be dissolved in a suitable solvent such as ethanol, to which is added an ethanolic solution of guanidine. In this embodiment, the ethanolic solution of guanidine may be derived by suspending guanidine hydrochloride in ethanol, adding sodium hydroxide (e.g. 50%) or any other suitable base and filtering off the precipitated sodium chloride. Alternatively, in a second embodiment, 3'-azido-3'-deoxythymidine may be suspended in water and solubilized by addition of sodium hydroxide (e.g. 50%) or any other suitable base until a pH of from about 12 to about 13 is reached. In this embodiment the concentration of the base used is not particularly critical provided at least about 1.0, more preferably from about 1.0 to about 1.1, molar equivalents of base are added. This generally results in a pH of the overall mixture in the range of from about 10 to about 14, preferably from about 12 to about 13. Thereafter, guanidine hydrochloride may be added to the 3'-azido-3'-deoxythymidine solution as a solid or as an aqueous solution.

Step (ii) in the present process comprises acidifying the precipitated salt produced in Step (i) to produce substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof. The choice of acid is not particularly restricted provided that it is capable of adjusting the pH of the mixture to less than about 9, preferably in the range of from about 1 to about 9, more preferably in the range of from about 6 to about 8, most preferably about 7. Preferably, acidification is done in an aqueous solvent, more preferably water. Non-limiting examples of acids suitable for use in Step (ii) include the hydrohalogens (chloride, bromide, iodide), acetic, sulfonic, sodium hydrogen sulfate, phosphoric, and monosodium phosphate acids. Preferred acids are hydrochloric acid, acetic and sulfonic acid. The most preferred acid is acetic acid.

In one embodiment of Step (ii), a volume of water equivalent to the weight of the salt produced is mixed with one equivalent of hydrochloric acid (e.g. 32% w/w) and heated to a temperature in the range of from about 70° to about 85° C. The salt is then added to and dissolved in this hot solution after which the pH of the solution is further adjusted, if necessary, to less than about 9, preferably in the range of from about 1 to about 9, more preferably in the range of from about 6 to about 8, most preferably about 7, by further addition of hydrochloric acid (e.g. 32% w/w) or any other suitable acid. More preferably, 1 equivalent of acetic acid is used at 70° C. to 95° C., giving a pH of approximately 6-7, and upon cooling crystallization will occur. Acetic acid permits the use of stainless steel reactors. Crystallization may be induced by cooling the pH-adjusted solution by about 5° C. During cooling, if crystals do not readily form, it is preferred to seed the solution at 45° C. As is known in the art of crystallization, seeding involves addition of a small amount of pure crystals of the compound to be reclaimed to a solution of the compound. Crystals of the compound then "grow" on the added seed.

The crystals of 3'-azido-3'-deoxythymidine or a derivative thereof may then be recrystallized in a conventional manner to yield substantially pure 3'-azido-3'-deoxythymidine or a derivative thereof. In Step (iii) of the present process, the relatively pure 3'-azido-3'-deoxythymidine or a derivative thereof may be recovered by an conventional physical separation technique such as filtration, decantation, evaporation and the like. Of course a combination of these techniques may also be used. If the recovered zidovudine is not substantially pure (i.e. purity of at least 99.5%), it may be subjected to further conventional purification techniques.

The mother liquor from crystallization in Step (ii) of the process typically contains a small amount of 3'-azido-3'-deoxythymidine or a derivative thereof. At least a portion of this residual amount may be recovered by adding sodium hydroxide (or any other suitable base) in a 1.0 or less, preferably a 0.5 or less, equivalent amount to the acid (e.g. hydrochloride) used. The resulting precipitate may be recycled to the initial stage of Step (ii) of the process.

The invention will be further described by reference to the following non-limiting specific examples:

EXAMPLE 1

A sample of impure 3'-azido-3'-deoxythymidine (18.76 g: 81.4% zidovudine and 18.6% 3'-N"-(3"'-azido-3"'-deoxythymid-3"-yl)-3'-deoxythymidine dimer was dissolved in hot methanol (57 mL). Guanidine hydrochloride (7 g, 0.0733 moles) was dissolved in methanol. It will be appreciated by those skilled in the art that the terms guanidine chloride and guanidine hydrochloride may be used interchangeably and describe the same compound. 50% caustic (5.6 g=0.07 moles) dissolved in methanol (17 mL) was added to the guanidine hydrochloride solution and the mixture was stirred for 30 minutes. After stirring, the mixture was filtered and the filtrate was added to the impure 3'-azido-3'-deoxythymidine solution. Crystals began to form after a few minutes. The mixture was stirred at 5 ° C. for 3 hours and then filtered, washed and dried to yield 15.56 g of precipitate comprising 3'-azido-3'-deoxythymidine guanidine salt.

Subjecting the precipitate to HPLC revealed that it comprised 93.05% 3'-azido-3'-deoxythymidine and 6.85% dimer. HPLC analysis of the mother liquor of the precipitate revealed that it comprised 59.1% zidovudine and 40.9% dimer. The mother liquor may be neutralized, preabsorbed on silica gel and applied to a silica gel column to obtain the dimer using ethylacetate-hexane (7:3) as the eluting solvent.

EXAMPLE 2

A sample of 3'-azido-3'-deoxythymidine (26.7 g) was suspended in water (119 mL), heated and basified with 50% caustic (8.94 g) to pH 12.5. Then guanidine hydrochloride. (11.5 g) dissolved in water (15 mL) was added in one portion. A precipitate formed in a few seconds and the mixture was stirred at 5° C. for 4 hours. The crystals were filtered and washed with water to yield 28.71 g of off-white crystals. Concentration of the mother liquor to about 1/3 volume yielded another 1.28 g of crystals.

EXAMPLE 3

A mixture of water (90 mL) and 32% hydrochloric acid (31.4 g) was heated at 75° C. 3'-azido-3'-deoxythymidine-guanidine complex (90 g) from Example 1 was added in portions to the aqueous solution and when the addition was completed the pH was 7.56. Thereafter, the pH was adjusted to 1.0 by further addition of 32% hydrochloric acid. The mixture was cooled and crystals formed spontaneously. After several hours the crystals were filtered to yield 61.64 g of crude 3'-azido-3'-deoxythymidine suitable for final purification. 50% caustic (22 g) was added to the mother liquor and a precipitate formed. After 30 minutes the precipitate was filtered to yield 7.99 g 3'-azido-3'-deoxythymidine-guanidine complex suitable for recycling (i.e. acidification).

EXAMPLE 4

A solution of impure 3'-azido-3'-deoxythymidine was prepared by a conventional process modeled after Glinski et at., supra, using 2,3 '-0-anhydro-5-O-tritylthyimidine (0.4 moles, 186.6 g) as a starting material. The composition of the major constituents in the reaction mixture can be found with reference to Composition A in Table 1. The theoretical mass of Composition A is 107 g.

Thereafter, Composition A was crystallized from solution to yield 77.97 g of crude 3'-azido-3'-deoxythymidine--see Composition B in Table 1.

The mother liquor from the recrystallization step was concentrated to yield another 3.25 g of 3'-azido-3'-deoxythymidine crystals--see Composition C in Table 1.

The 3'-azido-3'-deoxythymidine crystals from Compositions B and C were combined and purified using conventional techniques which included treatment with active charcoal to yield 62.93 g (0.235 moles, 58.9%) of pharmaceutical grade 3'-azido-3'-deoxythymidine.

The mother liquor from the production of Compositions B and C and the mother liquor from the production of pharmaceutical grade 3'-azido-3'-deoxythymidine were combined and evaporated to yield a syrup (approx. 34 g)--see Composition D in Table 1--which was then dissolved in ethanol (100 mL). Guanidine hydrochloride (15 g) was dissolved in ethanol (23 mL), 50% sodium hydroxide (11.5 g) was added and the resulting mixture was stirred for 10 minutes. The mixture was filtered and the precipitated sodium chloride was washed with ethanol (25 mL). The ethanolic guanidine and the ethanolic substrate solutions were separately heated to 75° C. and then combined with stirring resulting in the formation of a precipitate after about one minute. The mixture was cooled to 5° C., filtered and washed with ethanol to yield 21.62 g of 3'-azido-3'-deoxythymidine guanidine salt--see Composition E in Table 1. The resulting mother liquor from the production of Composition E was concentrated to about 1/3 of the original volume resulting in a yield of a further 1.23 g of 3'-azido-3'-deoxythymidine guanidine crystals--see Composition F in Table 1. The mother liquor from Composition F was thereafter subjected to HPLC--see Composition G in Table 1.

The 3'-azido-3'-deoxythymidine guanidine salts from each of Compositions E and F were combined and thereafter added to a mixture of water (23 mL) and 32% hydrochloric acid (7.98 g) which had been heated to 85° C. The salts were completely dissolved in a few minutes and the pH of the mixture was 8.2. The pH was then adjusted to pH 1 by further addition of 32% hydrochloric acid. The mixture was then cooled to ambient temperature and stirred for 16 hours. Thereafter, the mixture was further cooled to 5° C., stirred for 30 minutes, and filtered and washed with water to yield 15.74 g of substantially purified 3'-azido-3'-deoxythymidine--see Composition H in Table 1. The mother liquor from the production of Composition H was basified to pH 12.35 by addition of 50% sodium hydroxide to yield a precipitate which was filtered and washed with water to yield 2.18 g of 3'-azido-3 '-deoxythymidine guanidine salt--see Composition I in Table 1--which was of sufficient purity to be recycled with other batches of 3'-azido-3'-deoxythymidine guanidine salts (i.e. combine with salts from Compositions E and F). The mother liquor was acidified to pH 1 and extracted several times with methyl isobutyl ketone and the combined extract was set aside.

The crude 3'-azido-3'-deoxythymidine (15.74 g) from Composition H was purified by recrystallization from water after a charcoal treatment to yield 12.94 g (0.048 moles, 12%) of pharmaceutical grade 3'-azido-3'-deoxythymidine. Accordingly, the overall yield of pharmaceutical grade 3'-azido-3'-deoxythymidine was 70.9% which was representative of a 12% improvement over the prior art technique for producing 3'-azido-3'-deoxythymidine.

The mother liquor from the purification of Composition H was combined with the combined extract from Composition I and evaporated to about 10.4 g--see Composition J in Table 1. Silica column chromatography using first ethylacetate/hexanes 7:3, then ethyl acetate and finally ethanol yielded fractions of dimer (3'-N"-(3"-azido-3"'-deoxythymid-3"-yl)-3'-deoxythymidine)) which were crystallized from ethanol in two crops to yield 0.91 g. An NMR spectrum of the dimer (500 MHz-DMSO-d₆) yielded the following:

    ______________________________________                                         SHIFT (δ)                                                                          ASSIGNMENT                                                           ______________________________________                                         1.78 and 1.84                                                                            2 × 5, 3 × 3H, 2 x CH.sub.3                              2.15      DDD, 1H, J2.sub.1 '2.sub.2 '=13.9 Hz, J1'2.sub.1 '=7.3,                        J2.sub.1 '3'=11.9, H-2.sub.1 '                                       2.33      DDD, 1H, J2.sub.1 .sup.'" 2.sub.2 .sup.'" =13.6 J1.sup.'"                      2.sub.1 .sup.'" =6.1,                                                          J2.sub.1 .sup.'" 3.sup.'" =7.0, H-2.sub.1 .sup.'"                    2.43      DDD, 1H.sub.1, J1.sup.'" 2.sub.2 .sup.'" =6.1, J2.sub.2 .sup.'"                3.sup.'" =5.6, H-2.sub.2 .sup.'"                                     2.43      DDD, 1H, J.sub.1 .sup.'" 2.sub.2 '=7.3, J2.sub.2 '3'=5.8,                      H-2.sub.2 '                                                          3.49      DDD, 1H, J5.sub.1 '5.sub.2 '=11.7, J5.sub.1 '5'-OH=5.0,                        J5.sub.1 '4'=5.0, H-5.sub.1 '                                        3.56      DDD, 1H, J5.sub.2 '5-OH=5.0, J5.sub. 2 '4'=3, H-5.sub.2 '            3.59      DDD, 1H,J5.sub.1 .sup.'" 5.sub.2 .sup.'"=12.1, J5.sub.1 .sup.'"                5.sup.'" -OH=5.3,                                                              J5.sub.1 .sup.'" 4.sup.'" =3.9, H-5.sub.1 .sup.'"                    3.67      DDD, 1H, J5.sub.2 .sup.'" 5.sup.'" -OH=5.3, J5.sub.2 .sup.'"                   4.sup.'" =3.9, H-5.sub.2 .sup.'"                                     3.83      DT, 1H, J3.sup.'" 4.sup.'" =5.5, H-4.sup.'"                          4.11      M, 1H, J3'4'=3.4, H-4'                                               4.38      DT, 1H, H-3.sup.'"                                                   4.97      t, 1H, exchangeable, 5'-OH                                           5.23      t, 1H, exchangeable, 5.sup.'" -OH                                    5.57      DDD, 1H, H-3'                                                        6.14      t, 1H, H-1.sup.'"                                                    6.56      t, 1H, H-1'                                                          7.78      S, 2H, 2XH-6                                                         11.27     S, 1H, exchangeable, NH                                              ______________________________________                                    

The NMR assignments were made by comparison with the spectrum for 3'-azido-3'-deoxythymidine and by analysis of a 2-dimensional (COSY) experiment which allowed the assignment of the sugar protons to the appropriate furanose ring.

                  TABLE 1                                                          ______________________________________                                                  Content of Major Components (% by weight)                             Composition  1      2          3    4                                          ______________________________________                                         A            86.6   4.6        3.0  2.3                                        B            96.4   2.1        0.08 0.96                                       C            95.8   1.4        0.25 1.81                                       D            68.7   6.5        9.5  9.0                                        E            94.4   1.2        0.06 3.19                                       F            91.8   1.6        1.0  3.8                                        G            19.65  18.2       37.63                                                                               19.3                                       H            98.2   0.8        --   0.2                                        I            92.84  1.4        --   1.6                                        J            42.8   3.2        32.6 16.6                                       ______________________________________                                    

In Table 1, major components referred are as follows: ##STR6##

EXAMPLE 5

A basic solution (pH 12.5) of impure 3'-azido-3'-deoxythymidine in water (850 mL) was prepared by a conventional process from 0.8 moles of 2,3'-O-anhydro-5'-O-tritylthymidine. The major constituents in the basic solution are provided as Composition A in Table 2 wherein Compounds 1, 2, 3 and 4 are same as in Table 1 above for Example 6. Guanidine hydrochloride (92 g) was dissolved in water (200 mL) and added to the basic solution. After a few minutes a copious amount of precipitate was formed and the mixture was stirred at ambient temperature for 4 hours. The precipitate was filtered and washed with water to yield 221.19 g of product--see Composition B in Table 2.

The mother liquor from the production of Composition B was concentrated to about 1/2 the original volume resulting in precipitation of a solid. The solid was filtered and washed to further yield 12.84 g of 3'-azido-3'-deoxythymidine guanidine salt--see Composition C in Table 2. The mother liquor from production of Composition C was acidified to pH 1.0 and extracted several times with methyl isobutyl ketone and the extract was set aside.

A mixture of water (234 mL) and 32% hydrochloric acid (81.7 g) was heated to 85° C. and the combined 3'-azido-3'-deoxythymidine guanidine salt from Compositions B and C was added thereto. Within a few minutes, the salt was dissolved and the pH of the mixture was 6.0. The pH of the mixture was adjusted to 1.0 by addition of 32% hydrochloric acid. Thereafter, the mixture was cooled and stirred for 16 hours. The resultant crude 3'-azido-3'-deoxythymidine was filtered and washed to yield 152.39 g of crystals--see Composition D in Table 2. The mother liquor from production of Composition D was basified to pH 12.75 with 50% sodium hydroxide and stirred for 4 hours. The resultant crystals of crude 3'-azido-3'-deoxythymidine guanidine salt were filtered and washed to yield 2.96 g of product--see Composition E in Table 2--which was recycled as described hereinafter. The mother liquor from production of Composition E was extracted several times with methyl isobutyl ketone and the extracts were set aside.

The crude 3'-azido-3'-deoxythymidine (152.39 g) from Composition D was recrystallized from water by conventional techniques which included a charcoal treatment to yield 135.7 g (0.51 moles, 63.5%)of pharmaceutical grade zidovudine.

The two methyl isobutyl ketone extracts from treatment of the mother liquors of Compositions C and E were combined and evaporated to a syrup and combined with the mother liquor and further evaporated to 250 mL. The pH of this mixture was adjusted to 12.3 with 50% sodium hydroxide. The mixture was then heated to 70° C. and solid guanidine hydrochloride (16 g) was added and the temperature was increased to 95° C. and then allowed to cool. When the cooling mixture reached 65° C., a precipitate began to form. Cooling was continued to 5° C. when the mixture was filtered to yield 21.99 g of 3'-azido-3'-deoxythymidine guanidine salt--see Composition F in Table 2. The mother liquor from Composition F was acidified to pH 1.0 as above (Compositions C and E), extracted with methyl isobutyl ketone and set aside.

The 3'-azido-3'-deoxythymidine guanidine salt from Compositions E and F were treated in a manner similar to Compositions B and C above to yield 29.44 g crude 3'-azido-3'-deoxythymidine--see Composition G in Table 2. The mother liquor from Composition G was basified to pH 12.2. and the resultant crystals were filtered to yield 4.28 g of 3'-azido-3'-deoxythymidine guanidine salt--see Composition H in Table 2. The mother liquor from Composition H was acidified to pH 1.0 and extracted several times with methyl isobutyl ketone and set aside.

The crude 3'-azido-3'-deoxythymidine (29.44 g) from Composition G was purified to yield 26.7 g (0.1 moles, 12.5%) of pharmaceutical grade 3'-azido-3'-deoxythymidine. Accordingly, the overall yield of pharmaceutical grade 3'-azido-3'-deoxythymidine (162.4 g) was 76% which was representative of a 17.1% improvement over the prior art technique for producing 3'-azido-3'-deoxythymidine.

The mother liquor from the purification step was combined with the mother liquor extracts from Compositions F and H, and evaporated to a syrup, dissolved in ethanol, treated with ethanolic guanidine and the resultant precipitate filtered to yield 3.85 g of crude 3'-azido-3'-deoxythymidine guanidine salt--see Composition I in Table 2. The mother liquor from composition I was evaporated, the resulting residue was dissolved in water, the pH thereof adjusted to 1.0 and the resulting mixture was extracted several times with methyl isobutyl ketone. The extracts were combined and evaporated to yield 11.39 g of a thin syrup--see Composition J in Table 2. The syrup could be subjected to column chromatography to obtain the dimer (Compound 4), if desired.

An analytical grade sample of the 3'-azido-3'-deoxythymidine guanidine salt was prepared by suspending pharmaceutical grade 3'-azido-3'-deoxythymidine (53.4 g) in water (250 mL) and adding 50% sodium hydroxide (17.6 g) to provide a mixture having a pH of 13.0. The mixture was then heated to 70° C. Thereafter, guanidine hydrochloride (23 g) was added as a solid and the mixture was heated to 97° C. to dissolve the emerging precipitate. The mixture was then set aside to cool and crystallize, and it was stirred for 16 hours. Thereafter, the crystals were filtered and washed with water and ethanol to yield 58.76 (90%) of white crystals. The crystals had a melting point of 210.9°-212° C.

Analytical studies on the guanidine salt (C₁₁ H₁₈ N₈ O₄) yielded the following results:

    ______________________________________                                                    C    H          N      O                                            ______________________________________                                         Calculated:  40.49  5.56       34.34                                                                               19.61                                      Found:       40.92  5.45       34.17                                           ______________________________________                                    

U.V. Spectrum (H₂ O): λmax @268 nm

I.R. Spectrum (KBr disc): 2095 cm⁻¹ (indicated the presence of azide bond)

An NMR spectrum of the dimer (500 MH₂ -DMSO-d₆) yielded the following:

    ______________________________________                                         SHIFT (δ)                                                                            ASSIGNMENT                                                         ______________________________________                                         1.71        S, 3H, CH.sub.3                                                    2.12        m, 1H, H-2.sub.1 '                                                 2.25        m, 1H, H-2.sub.2 '                                                 3.57        m, 2H, H-5.sub.1 ',-5.sub.2 '                                      3.75        m, 1H, H-4'                                                        4.33        m, 1H, H-3'                                                        6.12        t, 1H, H-1'                                                        7.29        S, 1H, H-6                                                         7.38        broad singlet, 7H, exchangeable, 3-NH,                                         5'-OH, guanidine                                                   ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                  Content of Major Components (% by weight)                             Composition                                                                               1        2         3      4                                         ______________________________________                                         A          86.6     4.6       3.0    2.3                                       B          95.2     1.5       0.87   1.3                                       C          92.0     2.5       1.1    1.51                                      D          98.26    0.68      --     0.11                                      E          94.7     1.4       0.12   1.6                                       F          94.48    1.5       0.17   1.0                                       G          97.77    0.56      --     0.05                                      H          93.41    1.04      0.08   2.94                                      I          90.58    1.63      0.57   2.97                                      J          12.08    1.98      72.2   7.38                                      ______________________________________                                    

On this basis, the guanidine 3'-azido-3'-deoxythymidine salt was determined to have the following structure: ##STR7##

EXAMPLE 6

Zidovudine (3 g) was suspended in water (15 mL) and subjected to mild heat to yield a cloudy mixture. A base (1.2 equivalents)--see Table 3 for nature of base--was added to the mixture and a clear solution resulted. Thereafter guanidine hydrochloride (1.38 g) was added to the solution with stirring. Within a few minutes a precipitate formed (except in the cases of imidazole and pyridine). After one hour, the crystals were filtered, and washed with water and ethanol to yield the guanidine 3'-azido-3'-deoxythymidine salt confirmed by NMR. The exception to this was when the bases were imidazole and pyridine which resulted in the production of the free acid 3'-azido-3'-deoxythymidine (isolated and confirmed by NMR). As is evident from the results in this Example, aromatic heterocyclic amines such as imidazole are not useful in the context of a "base" in Step (i) of the present process.

EXAMPLE 7

A mixture of water (20 mL) and 32% hydrochloric acid--see Table 4 for amount used--was heated to 75 ° C. and water-damp guanidine 3'-azido-3'-deoxythymidine salt (105 g) having a water content of 38% by weight was added to the mixture. Heating was continued until all material had dissolved. The resultant pH of the solution was measured and then adjusted to a desired pH--see Table 4. The mixture was cooled to 60° C. and seeded. The mixture was further cooled to 25° C. and stirred for 2 hours resulting in the production of crystalline material. The crystals were filtered and washed to yield in all cases approximately the same amount of 3'-azido-3'-deoxythymidine crystals--see Table 4. In each case, the crystals were 99% pure and could be further purified to yield pharmaceutical grade 3'-azido-3'-deoxythymidine (purity of at least 99.5% ).

                  TABLE 3                                                          ______________________________________                                                       Zidovudine Guanidine                                             Base          Salt (g)       % Yield                                           ______________________________________                                         Methylamine   1.87           51.0                                              Isopropylamine                                                                               3.00           81.9                                              n-Butylamine  2.91           74.6                                              tert-Butylamine                                                                              2.97           81.3                                              Diethylamine  2.88           78.8                                              Diisopropylamine                                                                             3.10           84.6                                              Piperidine    2.69           73.6                                              Piperazine    2.11           57.7                                              Triethyl amine                                                                               2.81           76.9                                              Imidazole     0.00           0.0                                               Pyridine      0.00           0.0                                               ______________________________________                                    

                  TABLE 4                                                          ______________________________________                                                                             Zidovudine                                 Sample HC1 (g)  Resultant pH                                                                              Adjusted pH                                                                             (g)                                        ______________________________________                                         1      20.5     1.0        1.0      40.31                                      2      18.5     7.9        5.8      40.46                                      3      18.0     8.0        8.0      38.24                                      4      18.0     8.0        6.0      39.81                                      ______________________________________                                    

The mother liquors from each Sample were basified to pH 12-13 to yield 3'-azido-3'-deoxythymidine guanidine precipitate weighing from 4.5 g to 5.5 g. In each case, the crystals were 91% to 95% pure and could be further recycled to yield substantially pure 3'-azido-3'-deoxythymidine which in turn could be further purified to yield pharmaceutical grade 3'-azido-3'-deoxythymidine (purity of at least 99.5% ).

EXAMPLE 8

A mixture of water (34 mL) and acetic acid--see Table 5 for amount used--was heated to 75° C. and water-damp guanidine 3'-azido-3'-deoxythymidine salt (105 g) having a water content of 38% by weight was added to the mixture. Heating was continued until all material had dissolved. The resultant pH of the solution was measured and--see Table 5. The mixture was cooled to 60° C. and seeded. The mixture was further cooled to 25° C. and stirred for 2 hours resulting in the production of crystalline material. The crystals were filtered and washed to yield in all cases approximately the same amount of 3'-azido-3'-deoxythymidine crystals--see Table 5. In each ease, the crystals were 99% pure and could be further purified to yield pharmaceutical grade 3'-azido-3'-deoxythymidine (purity of at least 99.5%). The NMR of 3'-azido-3'-deoxythymidine isolated when 14.4 g of acetic acid was used showed an acetyl absorption of about 10%. Accordingly, not only is the use of a large excess of acid not required to achieve the same yield of 3'-azido-3'-deoxythymidine, in certain cases, the product may be affected.

                  TABLE 5                                                          ______________________________________                                                 Acetic Acid             Zidovudine                                     Sample  (g)          Resultant pH                                                                              (g)                                            ______________________________________                                         1       9.6          8.2        40.95                                          2       11.0         6.5        40.73                                          3       12.0         6.0        40.26                                          4       14.4         5.5        39.39                                          ______________________________________                                    

The mother liquors from each Sample were basified to pH 12-13 to yield 3'-azido-3'-deoxythymidine guanidine precipitate weighing from 4.5 g to 6.0 g. In each case, the crystals were 91% to 95% pure and could be further recycled to yield substantially pure 3'-azido-3'-deoxythymidine which in turn could be further purified to yield pharmaceutical grade 3'-azido-3'-deoxythymidine (purity of at least 99.5% ). 

We claim:
 1. A process for recovering 3'-azido-3'-deoxythymidine (AZT) or a salt thereof from a mixture comprising the steps of:(i) reacting AZT or a salt thereof in said mixture with a reagent selected from the group consisting of:(a) guanidine, (b) a guanidine salt in combination with at least about a 1.0 molar equivalent of a base selected from the group consisting of an alkali metal hydroxide; an alkaline earth metal oxide; an alkaline earth metal hydroxide; ammonium hydroxide; and an organic amine having the formula R¹ R² R³ N wherein R¹, R² and R³ are the same or different and are selected from the group consisting of C₁₋₁₀ alkyl groups and hydrogen, and (c) a combination of (a) and (b), to form a precipitated salt; (ii) acidifying said precipitated salt in a solvent to a pH of less than about 9 to yield free AZT; and (iii) recovering the AZT or salt thereof.
 2. The process of claim 1, wherein Step (i) comprises contacting a solution of AZT or a salt thereof with a solution of guanidine.
 3. The process of claim 2, wherein the solution of guanidine is derived by suspending guanidine hydrochloride in a solvent, adding a base thereto to produce a precipitate and filtering said precipitate to provide a filtrate consisting essentially of a solution of guanidine.
 4. The process of claim 3, wherein said base is sodium hydroxide and said precipitate is sodium chloride.
 5. The process of claim 3, wherein said solvent used for deriving the solution of guanidine is a polar solvent.
 6. The process of claim 5, wherein said solvent is ethanol.
 7. The process of claim 1, wherein Step (i) comprises contacting a suspension of AZT or a salt thereof with a base to dissolve the AZT or a salt thereof and to provide a solution having a pH of from about 10 to 14, and then adding guanidine hydrochloride to the solution.
 8. The process of claim 7, wherein said guanidine hydrochloride is in the form of a solid.
 9. The process of claim 7, wherein said guanidine hydrochloride is in the form of an aqueous solution.
 10. The process of claim 2, wherein Step (i) is conducted in the presence of a polar solvent.
 11. The process of claim 10, wherein the polar solvent is selected from the group consisting of water, methanol, ethanol, isopropanol, methylisobutyl ketone, and acetone.
 12. The process of claim 1, wherein in Step (ii) said solvent is an aqueous solvent.
 13. The process of claim 1, wherein in Step (ii) said solvent is water and hydrochloric acid.
 14. The process of claim 9, wherein said precipitated salt and solvent are heated to a temperature in the range of from about 70° to about 85° C.
 15. The process of claim 1, wherein Step (ii) further comprises adding said precipitated salt to a mixture of water and hydrochloric acid; heating the resulting mixture to a temperature in the range of from about 70° to about 85° C. to dissolve said precipitated salt; and adjusting the pH to a value of less than about
 9. 16. The process of claim 15, wherein the pH of said mixture is adjusted to a value in the range of from about 1 to
 9. 17. The process of claim 15, wherein the pH of said mixture is adjusted to a value in the range of from about 6 to about
 8. 18. The process of claim 15, wherein the pH of said mixture is adjusted to about
 7. 19. The process of claim 15, wherein Step (ii) further comprises cooling after pH adjustment thereof by at least 5° C. to induce formation of crystals of AZT or a salt thereof.
 20. The process of claim 19, wherein said AZT crystals are recrystallized to produce AZT or a salt thereof with a purity of at least 99.5%.
 21. The process of claim 3, wherein the alkali metal is selected from the group consisting of lithium hydroxide, sodium hydroxide, and potassium hydroxide.
 22. The process of claim 3, wherein the alkaline earth metal is selected from the group consisting of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide, and barium hydroxide.
 23. The process of claim 3, wherein the organic base is selected from the group consisting of methylamine, ethylamine, isopropylamine, n-butylamine, tertbutylamine, dimethylamine, diethylamine, diisopropylamine, triethylamine, piperidine, and piperazine.
 24. The process of claim 8, wherein prior to the addition of said guanidine hydrochloride in Step (i), the solution is heated to a temperature of about 70° C., said guanidine hydrochloride is thereafter added to the solution, and the temperature of the solution is increased to about 95° C.
 25. The process of claim 1, wherein acidifying in Step (ii) comprises the addition of at least one acid selected from the group consisting of hydrochloric acid, hydrobromic acid, hydroiodic acid, acetic acid, sulfonic acid, sodium hydrogen sulfate, phosphoric acid, and monosodium phosphate acid.
 26. The process of claim 1, wherein acidifying in Step (ii) comprises the addition of at least on acid selected from the group consisting of hydrochloric acid, acetic acid, and sulfonic acid.
 27. The process of claim 1, wherein the acidifying in Step (ii) comprises the addition of acetic acid. 3'-azido-3'-deoxythymidine 